The Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) radio interface supports both Frequency division duplex (FDD) and Time division duplex (TDD). A multiple input multiple output (MIMO) technique is an advanced antenna technique for a wireless device to improve spectral efficiency and thereby boost overall system capacity of 3GPP LTE systems. MIMO may be used for achieving diversity gain, spatial multiplexing gain and beamforming gain. Massive MIMO systems (i.e., 64+ antennas) are currently under investigation for 5G systems.
Ideal linear precoding uses full channel-state information (CST) at the transmitter. This may be possible only for TDD-based systems but not practical for FDD-based systems. Codebook based precoding, allows the receiver to explicitly identity a precoding matrix/vector based on a codebook that should be used for transmission. In the 3GPP LTE standard, separate codebooks are defined for various combinations of the number of transmit antennas and the number of transmission layers. The latter is also called rank information (RI).
For reporting RI and associated precoding matrix index (PMT), the user equipment (UE) uses an exhaustive search over all the precoding codebook elements. The exhaustive search is difficult to implement with the increased number of transmit antennas in massive MIMO systems. For example, in systems with four transmit antenna ports, the UE needs to search over all the 64 precoding entities for finding rank information and precoding index. Similarly, for 8 transmit antennas, the UE needs to search over all the 122 elements defined in the 3GPP standard. With large scale MEMO, the codebook size becomes larger. Thus, the UE complexity increases with additional transmit antennas deployed at the eNode B. Also, frequent CSI computation drains the UE battery by increasing power consumption, consuming more memory, and using additional processing units at the UE.